Sequoia, Joshua Tree Top List Of Most Polluted National Parks In US. Here Are Tips To Make Your Visit Better

California still has a lot of work to do before its national parks are free of pollutants: The four U.S. national parks with the highest ozone levels are all in California, according to a report from the National Parks Conservation Association.

The most affected areas are also some of California's most popular and accessible: Sequoia & Kings Canyon National Parks had the most air pollution of any park measured in the study, followed by Joshua Tree National Park, Mojave National Preserve, and Yosemite National Park.

The report looked at National Park Service data from 2021, measuring the level of manmade ozone pollution, hazy skies, and chemicals harmful to wildlife. Overall, 42 parks — about 10 percent of all NPS-managed areas where data was available — were found to have a "significant concern" of unhealthy air, the highest level of pollution as measured by the study.

California parks do a little better when it comes to the NPCA's measurement of haze and visibility. Sequoia is still the haziest park in the United States, but it's also the only park in California where the NPCA found a significant concern of visible pollution. Death Valley, Channel Islands and Pinnacles were the other national parks in the top 10 for visible pollution.

Why California parks are so polluted

According to the NPS, most of the pollution in these parks comes from elsewhere: pollution in Sequoia and Yosemite can originate from as far away as the San Francisco Bay Area, while westerly winds pushing pollution from the Los Angeles Basin are responsible for most of Joshua Tree's pollution. Motor vehicles, industrial emissions and consumer products are the most significant causes of ozone, according to the California Air Resources Board.

One silver lining from the report: Air quality is getting better in national parks across the country. Nationwide, the total number of parks with ozone levels classified as "unsatisfactory" decreased between 2019 and 2021, though most California parks have yet to see a significant reduction.

"Unfortunately, California is lagging behind much the rest of the country, and we've seen less improvements here in the California national parks," said Mark Rose, Sierra Nevada Program Manager for the NPCA.

Hot weather kicks up pollution

It's worth nothing that pollution levels have improved significantly over the last decades since the passage of the Clean Air Act Amendment in 1977 that protected air quality in national parks — and ozone levels aren't always high enough to be of concern. The winter, spring and early summer are a great time of year to get out and enjoy the cleaner air: Like California's urban areas, parks tend to be most polluted in hot weather.

"The hotter it gets, the more that ozone forms in the atmosphere," Rose said. "It impacts visibility and health in cities in California, and ozone forms and makes its way into our national parks during the hottest times of the year."

And while the majority of air pollution in the park originates elsewhere, the NPS has still committed to programs aimed at reducing ozone and other sources of poor air quality in parks. No matter where it originates from, these pollutants can harm native wildlife, especially plants, in addition to park visitors.

The NPS, who provided the data for the NPCA study, said that while they haven't reviewed the report's findings, they conduct their own monitoring to study the air pollutants in the park and work with other government agencies on air pollution regulation.

"The National Park Service is actively working to respond to climate change through different programs, including the Climate Change Response Strategy, Green Parks Plan, Planning for a Changing Climate, and the Resist Accept Direct framework," NPS spokesperson Scott Clemans wrote in a statement to LAist.

How to avoid unhealthy air

The NPS also offers these tips on avoiding air pollution in Sequoia and other parks:

• If you're visiting a mountainous park, air quality usually improves if you're high up enough. The NPS notes that elevations of 8500 feet and higher usually have clearer air even on days with bad air quality.
• Keep an eye on the air quality readings, as conditions can change and pockets of good air quality can spring up even on smoggy days.
• Check which areas of the park tend to be smoggiest and when. In Yosemite, for instance, mid-elevation west-facing slopes typically have more ozone, while sheltered areas like Yosemite Valley and Tuolumne Meadows tend to be less affected. Ozone levels usually peak when the wind is stronger in the afternoon and evening.